The present invention relates to a coated optical fiber including a coating layer formed by curing a resin on the outer periphery of an optical fiber by means of radiation of energy beams such as an ultraviolet ray, and a method of producing the same. In more detail, the present invention relates to the improvement in the coating resin of a coated optical fiber.
An optical fiber is used in such a manner that it is coated with a resin for the purpose of mechanical protection. Generally, to enhance productivity the coating resin is an energy beam curable resin which can be cured by radiation of ultraviolet ray or the like.
FIG. 1 is a schematic view showing the outline of an apparatus for manufacturing a general coated optical fiber. In FIG. 1, to manufacture a coated optical fiber 7, a wiredrawn optical fiber 3 is coated with an energy beam curable resin by a resin coating device 4 mainly consisting of a die and a point, and the curable resin is cured within a curing device 5 to thereby provide a single coating layer or a plurality of coating layers of resin, and then the optical fiber with such resin coating layer or layers is taken up by a take-up device 8.
FIG. 2 shows an example of a section of the coated optical fiber 7 manufactured in this manner. In general, the coated optical fiber has a double coating structure consisting of a buffer layer 10 and a protective layer 11 which are respectively made of an energy beam curable resin applied to and cured on the periphery of the optical fiber 3 composed of glass.
On the other hand, in recent years, with the spread of the amounts of production of optical fibers, the wire-drawing speeds of the optical fibers have been steadily increasing. For this reason, such a strong need exists for a resin to be used as a coating material that can be applied to the surface of the glass in good and uniform condition over a wide range of wiredrawing speeds.
Normally, if the wiredrawing speed of the fiber is increased, then there arises a phenomenon that the resin is difficult to have to attach to the glass because the fiber enters a die the first layer applied before the glass melted in a furnace is cool enough for the resin to attach to the glass.
Consequently, on the high speed side, the coating diameter of the resin is decreased and, as a result of this, the coating resin varies in thickness according to the wiredrawing speeds. In order to solve this problem, for example, in Japanese Patent Unexamined Publication No. 55-10470, there is proposed a method in which glass is forcibly cooled down prior to entrance into a die to thereby prevent a resin from being coated poorly, and the temperature of the glass is controlled to thereby control the covering diameter to a desired thickness.
However, if the glass is actually cooled down and is then wiredrawn at a high speed, then the covering diameter of the first coating layer tends to be larger than that obtained at a low speed. It is believed that this is because the pulling force of the glass is increased due to the increased wiredrawing speed and thus a larger quantity of resin is coated to the glass when compared with coating at the low speed.
As a result, in order to keep the covering diameter constant in a wide range of wiredrawing speeds, the temperature of the glass must be frequently and finely adjusted, which requires a very complicated controlling operation.